Posterior Cervical Triangle Study Notes

Posterior Cervical Triangle – Location & General Significance

• Also called “Lateral Cervical Region.”
• Clinically important for:
  • Passage of major neurovascular structures between neck, thorax, and upper limb.
  • Access/landmark for central line placement, brachial plexus blocks, lymph-node biopsy, fascial-space infection spread.
• Surface landmarks
  • Posterior border of sternocleidomastoid (SCM)
  • Anterior border of trapezius
  • Middle third of clavicle

Boundaries & Layers

• Borders
  • Anterior: Posterior border of SCM
  • Posterior: Anterior border of trapezius
  • Inferior (base): Middle \tfrac{1}{3} of clavicle
• Apex
  • Where SCM and trapezius meet at superior nuchal line of occipital bone.
• Roof
  • Skin → superficial fascia with platysma → investing layer of deep cervical fascia.
• Floor (muscular + fascial)
  • From inferior → superior: middle scalene, posterior scalene, levator scapulae, splenius capitis.
  • All enveloped by prevertebral fascia (part of deep cervical fascia).
• Fascial/visceral spaces in vicinity
  • Retropharyngeal space (between buccopharyngeal fascia and alar fascia); clinically the “danger space” for infection spread to mediastinum.
  • Carotid sheath (medial to triangle) encloses \text{CCA}, \text{IJV}, \text{CN X}.

Sub-Triangles Created by Omohyoid

• Inferior belly of omohyoid crosses the inferior part of the region.
  1. Occipital Triangle – superior & larger.
  2. Subclavian Triangle (a.k.a. omoclavicular / supraclavicular) – inferior & smaller; floor is first rib and slips of scalenes.

Musculature of the Region

• Superficial “wall” muscles
  • SCM (anterior wall)
  • Trapezius (posterior wall)
• Floor muscles (prevertebral group)
  • Splenius capitis: extends/rotates head; innervation – posterior rami mid-cervical nerves.
  • Levator scapulae: elevates, downwardly rotates scapula; innervation – dorsal scapular nerve C5 + cervical nerves C3–C4.
  • Scalenes
  • Anterior: attaches to 1st rib; flexes neck laterally & elevates 1st rib during forced inspiration; innervation – anterior rami C4–C6.
  • Middle: 1st rib; similar action; innervation – anterior rami cervical nerves.
  • Posterior: 2nd rib; innervation – anterior rami C7–C8.
  • Omohyoid (inferior belly): depresses hyoid; anchored to scapula; innervation – ansa cervicalis C1–C3.
  • Additional deep prevertebral muscles in proximal floor (but largely hidden from triangle):
  • Longus colli, longus capitis, rectus capitis anterior & lateralis (collectively flex head/neck, stabilize cervical spine).

Arterial Supply Traversing the Triangle

• Thyrocervical trunk branches (from 1st part of subclavian a.)
  • Transverse cervical artery (a.k.a. cervicodorsal trunk)
  • Passes superficial (lateral) to anterior scalene, phrenic nerve, & brachial plexus trunks.
  • Runs posterolaterally toward trapezius; usually bifurcates into
    • Superficial branch → trapezius.
    • Deep branch (dorsal scapular a.) → rhomboids, levator scapulae.
  • Suprascapular artery
  • Courses inferolaterally posterior to clavicle; crosses anterior scalene, phrenic nerve, 3rd part of subclavian a., cords of brachial plexus; continues over superior transverse scapular ligament to supraspinous fossa.
• 3rd part of Subclavian artery
  • Lies on 1st rib, posterior to anterior scalene; anterior to inferior trunk of brachial plexus.
• Occipital artery
  • Branch of external carotid; ascends at apex near SCM-trapezius junction; supplies posterior scalp & muscles.

Venous Drainage

• External Jugular Vein (EJV)
  • Formed by posterior auricular + posterior retromandibular vv.; runs superficial to SCM but deep to platysma → pierces investing fascia → empties into subclavian v. superior to clavicle.
  • Drains scalp, lateral face.
• Subclavian Vein
  • Continuation of axillary v.; runs anterior to anterior scalene; joins IJV to form brachiocephalic v. posterior to medial clavicle.
• Tributaries commonly seen in triangle: suprascapular v., cervicodorsal v., occipital v.

Nervous Structures Within / Crossing Triangle

Motor Nerves

• Spinal Accessory Nerve (CN XI)
  • Emerges at junction of upper & middle thirds of SCM’s posterior border → crosses triangle obliquely → penetrates trapezius.
  • Injury (e.g., lymph-node biopsy) ⇒ shoulder droop, inability to shrug/abduct beyond \sim90^\circ.
• Brachial Plexus – trunks (roots C5–T1)
  • Emerge between anterior & middle scalenes (interscalene gap) → travel inferolaterally.
  • Superior (upper) trunk C5–C6
  • Gives suprascapular n. C4–C6 & nerve to subclavius C4–C6.
  • Middle trunk C7
  • Inferior (lower) trunk C8–T1 (posterior/inferior in triangle).
• Supraclavicular branches (motor) from brachial plexus
  • Dorsal scapular n. C4–C5: pierces middle scalene → rhomboids, levator scapulae.
  • Long thoracic n. C5–C7: posterior to brachial plexus → serratus anterior; lesion ⇒ winged scapula.
  • Nerve to subclavius C5–C6 (often with C4); sometimes provides accessory root to phrenic nerve.
• Phrenic nerve C3–C5
  • Motor, sensory, sympathetic to diaphragm, pericardium, mediastinal pleura.
  • Lies on anterior scalene deep to prevertebral fascia, lateral to IJV, posterior to SCM.

Cervical Plexus (ventral rami C1–C4)

• Located anteromedial to levator scapulae & middle scalene, deep to SCM.
• Looped pattern from ascending/descending branches of adjacent levels.

Sensory Branches – emerge at “nerve point of neck” (Erb’s point)

• Lesser occipital n. C2: scalp posterior to auricle.
• Great auricular n. C2–C3: parotid region, ear, angle of mandible.
• Transverse cervical n. C2–C3: anterior cervical skin.
• Supraclavicular nn. C3–C4: clavicular, shoulder, upper thoracic skin (medial, intermediate, lateral groups).

Additional Deep (motor) Branches

• Ansa cervicalis C1–C3: strap muscles (omohyoid, sternohyoid, sternothyroid).
• Nerves to prevertebral muscles (longus colli, longus capitis, rectus capitis ant./lat., scalenes).

Tables – Key Muscle Attachments, Innervation, Action (summary)

• Anterior vertebral muscles
  • Longus colli: bodies C1–T3 ↔ transverse processes C3–C6; flex neck, rotate contralaterally.
  • Longus capitis: transverse processes C3–C6 ↔ basilar occipital bone; flex head.
  • Rectus capitis anterior: lateral mass of atlas ↔ occipital bone anterior to condyle; flex/stabilize head.
  • Anterior scalene: transverse processes C3–C6 ↔ 1st rib; flex neck, elevate 1st rib.
• Lateral vertebral muscles
  • Rectus capitis lateralis: transverse process atlas ↔ jugular process occiput; lateral flex/stabilize head.
  • Splenius capitis, levator scapulae, middle & posterior scalene: see above.

Relationships Demonstrated in Cross-Section (C7 Level)

• Skin → platysma → investing fascia → SCM & trapezius.
• Retropharyngeal space posterior to pharynx, anterior to alar fascia/longus colli.
• Carotid sheath lateral to pharynx; brachial plexus & subclavian a. posterior to clavicle.

Clinical Correlations / Practical Points

• Boundaries used for palpation and anesthetic blocks (e.g., interscalene brachial plexus block at Erb’s point targets roots/trunks).
• External jugular vein prominence may indicate heart failure or SVC obstruction; provides landmark for central venous access.
• Fascia directs spread of infections: prevertebral fascia → retropharyngeal abscess; alar space extension to mediastinum.
• Knowledge of spinal accessory nerve path essential during lymph-node excision to prevent trapezius palsy.

Mnemonics & Memory Aids

• "Order of structures superficial → deep across omoclavicular triangle near clavicle: Skin – Platysma – EJV – Investing fascia – Omohyoid – Subclavian Vein – Anterior Scalene – Subclavian Artery – Brachial Plexus" → remember as “S-P-E-I-O-V-A-A-B – ‘Speio Vab’ ”.
• Nerve roots for phrenic: C3, C4, C5 "keep the diaphragm alive."
• Brachial plexus trunks arrangement superior to inferior matches spinal root order C5/6 – C7 – C8/T1 (same as traffic-light: green-yellow-red descending).